Showing papers on "Glass transition published in 1991"

Amorphous Zr–Al–TM (TM=Co, Ni, Cu) Alloys with Significant Supercooled Liquid Region of Over 100 K

Abstract: Amorphous alloys exhibiting a wide supercooled liquid region above 100 K were found to form in a compositional range from 0 to 3%Co, 0 to 15%Ni and 10 to 23%Cu in Zr 65 Al 7.5 Cu 2.5 (Co 1-x-y Ni x Cu y ) 25 system by melt spinning. The temperature span ΔT x (=T x -T g ) between glass transition temperature (T g ) and crystallization temperature (T x ) reaches as large as 127 K for Zr 65 Al 7.5 Ni 10 Cu 17.5

Plasticizing Effect of Water on Thermal Behavior and Crystallization of Amorphous Food Models

Abstract: Dehydrated sugar solutions were used as models of thermal behavior of amorphous foods, and of the effect of temperature, moisture content and time on physical state of such foods. The transition temperatures determined were glass transition (Tg), crystallization (Tcr) and melting (Tm) which all decreased with increasing moisture. Tg of a sucrose/ fructose model had a slightly lower value than the empirical “sticky point,” at all moisture contents studied. Crystallization of sucrose was delayed by addition of fructose or starch. Crystallization above Tg was time-dependent, and the relaxation time of this process followed the WLF equation.

Mg–Cu–Y Amorphous Alloys with High Mechanical Strengths Produced by a Metallic Mold Casting Method

Abstract: A low pressure casting of Mg-Cu-Y melts into copper molds was found to cause amorphous bulks in a cylindrical form. The maximum diameter of the amorphous cylinder (D c ) shows a significantcompositional dependence and reaches a maximum value of 4.0 mm for Mg 65 Cu 25 Y 10 . The compositional dependence of the D c is similar to that for the temperature span of the supercooled liquid region, ΔT x (=T x -T g )

The glass transition of organic liquids confined to small pores

Abstract: The glass-transition temperatures, Tg, of organic liquids confined to small pores were studied by differential scanning calorimetry (DSC). The Tg was measured as a function of pore size in controlled pore glasses (CPG) having pore diameters in the range of 40–730 A. The surface of the glass was treated with hexamethyldisilazane to promote wetting by the organic liquids studied (o-terphenyl and benzyl alcohol). Glasses formed in the pores had a lower Tg than in the bulk and the reduction in Tg increased as the pore size decreased. For example, the depression of the glass transition temperature, ΔTg, of benzyl alcohol in 40 A and 85 A pores was 7.2 K and 3.1 K, respectively. The magnitude of ΔTg also depends on the material; e.g. for o-terphenyl in the 85 A pores, ΔTg was 8.8 K versus 3.1 K for benzyl alcohol. In general, it was noted that ΔTg was considerably less than for the depression of the crystalline melting point, ΔTm, studied in related work. For example, for benzyl alcohol in the 85 A pores, ΔTm was ∼ 25 K and ΔTg was ∼ 3 K.

Water and Molecular Weight Effects on Glass Transitions in Amorphous Carbohydrates and Carbohydrate Solutions

Abstract: The effects of water, freeze-concentration and effective molecular weight (M,) on glass transition (T,) of maltose and maltodextrins were studied, and methods to predict Tg were used to establish state diagrams. Tg of maximally freeze-concentrated solutes (Tg and onset of ice melting (TJ increased with M,, and for high molecular weight polysaccharides TL and TA were predicted to have the same temperature value. Ice formation at TL

Liquides, cristallisation et transition vitreuse = Liquids, freezing and glass transition : Les Houches, session LI, 3-28 juillet 1989

Abstract: Part I: 1. The structure of simple liquids (I. McDonald). 2. Statistical mechanics of cellular automata fluids (M.H. Ernst). 3. Crystallization of liquids: a density functional approach (D. Oxtoby). 4. Theory of quantum processes in liquids (D. Chandler). 5. Aspects of structural glass transitions (W. Gotze). Part II: 6. Interfacial phenomena (B. Widom). 7. Molecular motion in liquids (P.A. Madden). 8. Neutron scattering and collective dynamics in liquids and glass (F. Mezei). 9. Statistical mechanics of liquid crystals (D. Frenkel). 10. Colloidal suspensions (P.N. Pusey). Seminars: 1. Computer simulation of equilibrium nonequilibrium molecular dynamics (G. Ciccotti). 2. Neutron scattering investigation of the structure of disordered materials (P. Chieux). 3. The physics of liquid and amorphous water (J. Teixeira). 4. Crystallization phenomena in colloidal dispersions (H.N.W. Lekkerkerker). 5. Transport and relaxation phenomenology near the glass transition - assessment of theory (C.A. Angell). 6. Exact results for the liquid-solid interface (L. Blum and D.A. Huckaby). 7. Wetting and spreading: some experiments (L. Leger).

Applying state diagrams to food processing and development.

Abstract: The physical state of food components affects their properties during processing, storage, and consumption. Removal of water by evaporation or by freezing often results in formation of an amorphous state (Parks et al., 1928; Troy and Sharp, 1930; Kauzmann, 1948; Bushill et al., 1965; White and Cakebread, 1966; Slade and Levine, 1991). Amorphous foods are also produced from carbohydrate melts by rapid cooling after extrusion or in the manufacturing of hard sugar candies and coatings (Herrington and Branfield, 1984). Formation of the amorphous state and its relation to equilibrium conditions are shown in Fig. 1 [see text]. The most important change, characteristic of the amorphous state, is noticed at the glass transition temperature (Tg), which involves transition from a solid "glassy" to a liquid-like "rubbery" state. The main consequence of glass transition is an increase of molecular mobility and free volume above Tg, which may result in physical and physico-chemical deteriorative changes (White and Cakebread, 1966; Slade and Levine, 1991). We have conducted studies on phase transitions of amorphous food materials and related Tg to composition, viscosity, stickiness, collapse, recrystallization, and ice formation. We have also proposed that some diffusion-limited deteriorative reactions are controlled by the physical state in the vicinity of Tg (Roos and Karel, 1990, 1991a, b, c). The results are summarized in this article, with state diagrams based on experimental and calculated data to characterize the relevant water content, temperature, and time-dependent phenomena of amorphous food components.

New polymers by homogenous zirconocene/aluminoxane catalysts

Abstract: With homogenous catalysts on the basis of chiral metallocenes and methylaluminoxane it has become possible to polymerize cyclic olefins like cyclobutene, cyclopentene or norbornene. No ring opening reaction occurs. The crystalline polycycloalkenes show extremly high melting points between 400 and 600°C. Copolymers of cyclopentene with propene are amorphous. They have a low molecular weight and glass transition point. On the other hand the glass transition temperature of norbornene/ethene copolymers exceeds 130°C. These copolymers could be used as materials for optical discs and fibers.

Dynamic light-scattering study of the glass transition in a colloidal suspension.

Abstract: This paper describes a light-scattering study of the glass transition in nonaqueous suspensions of sterically stabilized colloidal spheres. The observed phase behavior, fluid, crystal, and glass, is consistent with an essentially hard-sphere interaction between the particles. Metastable fluid states were obtained upon shear melting the crystalline phases by tumbling the samples. Their intermediate scattering functions, measured by dynamic light scattering, showed the emergence of a nondecaying component, implying structural arrest, at essentially the same concentration as that at which homogeneously nucleated crystallization was no longer observed. The overall forms of the intermediate scattering functions are consistent with the predictions of mode-coupling theories for the glass transition. Supplementary studies of the static structure factors indicated only short-ranged spatial order for particle concentrations ranging from the equilibrium fluid through the metastable fluid to the glass.

Glass transition temperature as a function of conversion in thermosetting polymers

Abstract: A simple equation to model the dependence of the glass transition temperature (T g ) on the extent of reaction in highly cross-linked thermosetting polymers has been derived. The model assumes that the increase in T g is caused by (a) decrease in chain-end concentration, (b) formation of effective cross-links, and (c) further decrease in the configurational entropy due to departure from Gaussian behavior at high cross-link densities

Premelting crystalline relaxations and phase transitions in nylon 6 and 6,6

Abstract: Variable-temperature XRD and NMR measurements show that nylon 6 (N6) undergoes crystalline relaxations between the glass transition temperature and the melting point. These relaxations bring about a crystalline transition between 80 and 170 o C from a monoclinic structure to a new crystalline structure, which is also most likely monoclinic. Thus, the presence of a Brill transition in N 6 is demonstrated for the first time; this phenomena has been extensively studied in nylon 6,6 (N6,6)

Oxidation of methyl linoleate encapsulated in amorphous lactose-based food model

Abstract: A food model was used to study the effect of physical changes in the amorphous matrtix on the rate of lipid oxidation. The glass transition temperature (T g ) and crystallisation behaviour of the material were determined by using differential scanning calorimetry. The rate of oxidation was measured spectrophotometrically in samples incubated at various temperatures above T g .

A food polymer science approach to structure-property relationships in aqueous food systems: non-equilibrium behavior of carbohydrate-water systems.

Abstract: Descriptions of the functional significance of carbohydrates based on the familiar equilibrium thermodynamics of very dilute solutions fail for pragmatical time scales and conditions, which are far from equilibrium. This is not too surprising, since limiting partial-molar properties reflect the independent behavior of solute in the limit of infinite dilution where free volume is maximum at a given temperature, while Tg′-Wg′ properties reflect the cooperative behavior of solute-plasticizer blends at the limiting minimum value of free volume to observe relaxation within experimental time scales. Carbohydrate-water systems, with well-characterized structure and MW above and below the entanglement limit, provide a unique framework for the investigation of non-equilibrium behavior. Thermal analysis by DSC reveals the central role of water as a plasticizer for carbohydrates and of the glass transition as a physicochemical parameter that governs their properties, processing, and stability. A classical polymer science approach is used to study structure-property relationships of carbohydrates as water-compatible food polymers, which are treated as homologous systems of polymers, oligomers, and monomers with their plasticizers and solvents. Mechanical relaxation behavior is described by a “transformation map” of the critical variables of moisture content, temperature, and time. The glass curve is a reference contour, which represents the limiting isogram for free volume, local viscosity, relaxation rates, and rotational and translational mobility. Map domains are discussed as aspects of “water dynamics,” to dispel the myth of “bound water,” and “glass dynamics,” to relate to macroscopic structure and collapse phenomena. A particular glass with invariant composition and Tg (prepared by freeze-concentration) is identified as a pivotal and practical reference state. The Tg observed during DSC analysis is often an effective Tg, resulting from instantaneous relative relaxation rates and non-uniform distribution of total sample moisture. Non-equilibrium melting, annealing, and gelation/recrystallization of kinetically metastable, partially crystalline carbohydrate systems exhibit non-Arrhenius kinetics which depend on the magnitude of ΔT above the appropriate Tg, as defined by WLF relaxation transformations. Thermally reversible aqueous gels (crystallized from an under-cooled, rubbery melt) are described by a “fringed micelle” structural model for a three-dimensional polymer network, composed of microcrystalline junction zones crosslinking plasticized amorphous regions of flexible-coiled, entangled chain segments.

Glass transitions in polymer/co2 mixtures at elevated pressures

Abstract: It is shown that poly(methyl methacrylate) and polystyrene undergo glass transitions in the presence of CO 2 at moderate pressures (<100 atm). The observed T g depressions at these moderate pressures are quite large and are directly related to the amount of CO 2 sorbed by the two polymers. In contrast, the CO 2 pressure at the glass transition is related to both the amount of CO 2 sorbed and the temperature at the glass transition. At substantially higher CO 2 pressures (i.e., more than several hundred atmospheres), a hydrostatic pressure effect must also be considered

Is There a Growing Correlation Length near the Glass Transition

Abstract: We present the results of a molecular-dynamics study on a Lennard-Jones mixture. Measurements of density and bond-angle correlation functions show that the answer to the question posed in the title is NO. We also show that icosahedral order plays no special role in the glass transition, but that a normalized measure of the persistence of orientational correlations in the glass shows a systematic dependence on the spherical harmonic index l.

Dynamical heterogeneity in a miscible polymer blend

Abstract: The dynamic mechanical spectra in the glass transition region of miscible blends show an extraordinaryly broad transition zone when the blend has a high concentration of poly(vinylethylene). The application of the coupling model to the data suggests that this breadth is a manifestation of an inhomogeneous distribution of relaxation times and coupling parameters engendered by concentration fluctuations. A consequence of this dynamical heterogeneity is deviation from the principle of time-temperature superpositioning for miscible polymer mixtures in the glass transition region

Glass transitions in soybean seed : relevance to anhydrous biology.

Abstract: We have investigated the mechanism by which anhydrobiotic organisms can survive severe dehydration. The method used was measurement of the rotational diffusion coefficient of a hydrophilic spin probe, inserted in the cytoplasm of soybean (Glycine max L.) axes, as a function of temperature and sample water content. Results indicate the existence of a hydration-dependent glass-like transition at physiological temperatures. No glass transitions have been observed in desiccation-intolerant samples, suggesting that the ability to withstand dehydration is associated with glass formation.

A structural basis for ionic diffusion in oxide glasses

Abstract: The relationships between the environments of cations in alkali silicates measured by X-ray absorption fine structure (XAFS) and magic angle spinning nuclear magnetic resonance (MASNMR) are considered. Both are consistent with the modified random network for glass structure in which modifiers form channels percolating through the network. It is proposed that the mechanisms determining the distribution of bridging and non-bridging oxygen atoms at the glass transition are the same as those that promote ionic transport at lower temperatures in the glass. In particular the results of XAFS and MASNMR can be used to predict the activation energy for ionic transport and the magnitude of the electrical conductivity. Values of these parameters for alkali disilicates are in good agreement with those measured directly from transport properties.

Molecular dynamics simulation of local chain motion in bulk amorphous polymers. I. Dynamics above the glass transition

Abstract: The local chain dynamics of bulk amorphous polymers above the glass transition temperature is studied by means of molecular dynamics simulations in the time scale up to 10−9 s. A model of polyethylene having a realistic density is used where a fully vibrational treatment is applied to the chains subject to nonbonded interactions between segments. Also used for comparison is a freely‐rotating chain model with no torsional potential, but otherwise having the same features. Time‐correlation functions of various vectors embedded in the polymer backbone are evaluated and the result shows a strong dependence of the relaxation on the direction of the vector in both models. The chain reorientation process is then analyzed in terms of the two separate components of the motion: one, the reorientation of the chain axis and the other, the rotation of the chain around it. On a short time scale, the reorientation process is affected directly by the conformational transitions. On the longer time scale, however, the rela...

Nanocrystalline materials by crystallization of metal-metalloid glasses

Abstract: The formation of ultrafine microstructures by crystallization of metal-metalloid glasses was investigated by means of electron microscopy as well as in situ time-resolved X-ray diffraction. The results can be understood on the basis of nucleation and growth theories, taking into account the effect of recalescence during massive crystallization and the differences in the mode of crystallization and the diffusivity. In a polymorphic crystallizing Fe66Ni10B24 glass the finest microstructure can be achieved by annealing at temperatures significantly below the “nose” of the TTT diagram; the finest grain size can be calculated and observed to be in the range of about 0.1 μm. In glassy Fe73.4Cu1Nb3.1Si13.4B9.1 (FINEMENT) the combination of a reduced growth rate due to the niobium content as well as with increasing size of the primary crystals and an accelerated nucleation rate due to the copper additions allows the formation of extremely fine-grained microstructures in primary crystallizing metal-metalloid glasses at temperatures above the glass transition.

Phase Transitions of Amorphous Sucrose and Frozen Sucrose Solutions

Abstract: Amorphous sucrose showed typical thermal transitions of amorphous materials. The temperatures of those transitions, glass transition, crystallization and melting, decreased with increasing moisture content. The glass transition temperature of dry amorphous sucrose at 57°C was decreased to -46°C as the sucrose was plasticized with excess water. That value remained constant due to maximal freeze-concentration with 72-73% sucrose in the unfrozen matrix which showed ice melting at -34°C. Concentrated solutions may become supercooled resulting in partial freeze-concentration with resultant lowering of glass transition temperature.

Pressure-sensitive adhesives based on preferentially tackified immiscible elastomers

Abstract: A pressure sensitive adhesive of a mutually immiscible first elastomer (styrene-butadiene block copolymer) exhibiting a first glass transition temperature and a second elastomer (e.g., a styrene-isoprene-styrene block copolymer) exhibiting a second glass transition temperature greater than the first is tackified by a tackifying system comprising tackifier preferentially miscible with the second elastomer to provide a dynamic mechanical spectrum plot of tangent delta versus temperature in °C. showing two separate and distinct glass transition temperature peaks. The mixture exhibits excellent processability and ambient and low temperature adhesive properties.

Gibbs-DiMarzio equation to describe the glass transition temperature trends in multicomponent chalcogenide glasses

Abstract: High purity chalcogenide glasses incorporating Ge, Sb, Se, As and Te were prepared by vacuum melting of previously distilled 5 N to 6 N pure raw materials from which the surface oxide was also removed in some instances. Glass transition temperatures, Tg, of several chalcogenide glasses prepared by similar processing techniques were determined using differential scanning calorimetry (DSC). An emperical relationship between the glass transition temperature and average coordination number is proposed by modifying the Gibbs-DiMarzio equation for glass transition of a cross-linked polymer as a function of cross-link density. The results for the Tg of three ‘iso-structural’ systems are reported and discussed in light of presumed structural arrangements in these glasses.

Characteristic length of glass transition: experimental evidence

Abstract: The glass transition length scale determined from formulae of a fluctuation approach (2ξa = 2 ... 5 nm) is confirmed by two independent experimental methods. Firstly the glass transition parameters in self-organized amorphous layers of variable thickness in semicrystalline poly(ethylene terephtalate) show significant sensitivity to the nanometer length scale of ξa. The results can be explained by a direct influence of spatial limitations on the large mode lengths of the glass transition. Secondly, the analysis of the arrangement of different system responses across the glass transition zone at constant temperature in polystyrene and poly(vinyl acetate) results in the location of ξa between the limits 0.6 nm (from local modes) and 10 nm (entanglement spacing). Experimental temperature dependencies of spectral widths are discussed with those obtained from the fluctuation approach. The report also contains a discussion of the presumed general scaling principle: At a given state, short mode length corresponds to high frequency (mobility) and vice versa.

The study of the glass transition of glutenin using small amplitude oscillatory rheological measurements and differential scanning calorimetry

Abstract: Hydrated glutenin was studied using mechanical spectrometry and differential scanning calorimetry (DSC). Small amplitude oscillatory measurements showed, as a function of temperature, that hydrated glutenin between 4% and 14% moisture content showed glass transition temperatures between 132 and 22 °C. Over the same moisture range, glutenin samples had glass transitions temperatures between 110 and 21 °C as measured by DSC. Both techniques showed that the glass transition temperature of glutenin shifted to lower temperatures with increasing moisture content. Linear approximations of the glass transition temperatures obtained by DSC vs moisture content showed a depression of about 9 °C/wt. %. Both methods showed that glutenin was very sensitive to the plasticizing effect of water in the moisture range studied and appears to be an amorphous, water plasticizable polymer.